One conventional example of the above-described filler plug for the hydraulic device is shown in FIG. 5 (see PTL 1, for example). As shown in FIG. 5, a filler plug 1 for a hydraulic device includes an external thread portion 2. A head portion 3 having a larger diameter than the external thread portion 2 is formed on an upper end portion of the external thread portion 2. An engaging recess 4 is formed on an upper end surface of the head portion 3. The engaging recess 4 is, for example, a hexagon socket.
As shown in FIG. 5, when sealing an opening end portion of an oil passage 5 (through hole) formed in the hydraulic device by using the filler plug 1, a hexagonal wrench (tool) is caused to engage with the engaging recess 4 formed on the head portion 3 of the filler plug 1, and the external thread portion 2 of the filler plug 1 is tightened with respect to an internal thread portion 6 formed in the oil passage 5 of the hydraulic device. With this, a jaw-shaped portion 7 formed at a lower portion of the head portion 3 of the filler plug 1 can be caused to strongly contact a funnel-shaped inclined inner peripheral surface 8 formed at the opening end portion of the oil passage 5. Thus, the opening end portion of the oil passage 5 can be sealed by using the filler plug 1.
As shown in FIG. 5, an axial cross-sectional shape of the jaw-shaped portion 7 of the filler plug 1 is a circular-arc shape having a curvature radius R1 (center O). Therefore, even if a taper angle αl of the funnel-shaped inclined inner peripheral surface 8 varies, the jaw-shaped portion 7 can be caused to closely contact the inclined inner peripheral surface 8. Thus, the opening end portion of the oil passage 5 can be surely sealed.
Here, for example, when the pressure in the oil passage 5 shown in FIG. 5 is high, it is necessary to tighten the external thread portion 2 of the filler plug 1 with respect to the internal thread portion 6 at a high tightening torque to cause the jaw-shaped portion 7 of the filler plug 1 to further strongly contact the inclined inner peripheral surface 8 formed at the opening end portion of the oil passage 5. However, if the tightening torque is high, the engaging recess 4 collapses. In order to avoid this, it is necessary to decrease contact pressure, and it is necessary to increase an engaging force between the tool and the engaging recess 4 in a tightening direction by increasing a depth d1 of the engaging recess 4 of the filler plug 1 with which the tool engages and a height h1 of the head portion 3.